1. Field of the Invention
This invention relates generally to low-pass filters and, more particularly, to a reversible, odd-order low-pass microfilter for separating DSL and home networking signals from voice-band signals existing on the same pair of copper wires.
2. Description of the Background Art
With the advent of DSL (Digital Subscriber Line) and home networking data transmission technologies, it may be desirable to have DSL signals, home networking signals, or both present on a home telephone wiring network simultaneously with voice-band signals. Voice-band signals are commonly referred to as POTS (Plain Old Telephone Service) signals. Providing DSL service, home networking, and POTS over standard telephone lines permits the home telephone wiring network to operate as a local area network (LAN), while at the same time permitting voice-band and DSL service to be transmitted across the home telephone wiring network.
Despite the advantages of providing DSL, home networking, and POTS signals simultaneously over a common home telephone wiring network, it is desirable to prevent energy from the DSL and/or home networking signal carriers from reaching voice-band, or POTS, appliances coupled to the home telephone wiring network. It is also desirable to prevent POTS device impedance effects beyond about 4 kHz from entering onto the home telephone wiring network and disrupting transmission of DSL data signals. Voice-band appliances may include, for example, telephone sets, facsimile machines, 56K modems, and the like. Indeed, energy from the DSL or home networking signal carriers may cause nonlinear behavior of the voice-band appliances to create noise into the POTS connection. Further, preventing DSL and home networking signals from reaching voice-band appliances protects the DSL and home networking transports from high-frequency inter-modulation products of the voice-band appliances.
Voice-band appliances typically undergo impedance changes during operation. For example, state changes in a POTS device such as on/off hook, dialing, and ringing tend to affect the impedance of the POTS device. This change in impedance, unless isolated from the DSL modem, may limit the throughput of the DSL or home networking devices and may require dynamic bit reloading in modulation and line retraining, and could result in loss of modem connection.
Conventionally, a second-order low-pass Butterworth filter is disposed between the home telephone wiring network and an associated POTS device to prevent DSL signals, such as ADSL signals, on the home telephone wiring network from entering the POTS device and to prevent transient noise from POTS devices from interfering with the proper operation of a DSL modem coupled to the home network and vice versa. The filter topology of the second-order Butterworth microfilter is inherently asymmetrical and generally includes one coupled inductor (or two uncoupled inductors) and one capacitor. This design is unilateral and non-reversible in that it requires, for proper operation, that the microfilter be oriented between the POTS device and the home telephone wiring network such that the coupled inductor is disposed between the home telephone wiring network and the capacitor. Indeed, if the capacitor is disposed adjacent to the home telephone wiring network, high frequency signals, such as DSL signals, on the home telephone wiring network are likely to short, or be shunted, across the capacitor, thus interfering with the operation of the DSL modem. In short, these conventional microfilters are not reversible in that they only function properly when correctly oriented. Thus, users who install the two-pole microfilter in a reversed, or "backwards", configuration will likely suffer from poor filter and DSL modem performance.
Another disadvantage of conventional second-order Butterworth microfilter designs is that they do not provide sufficient attenuation of DSL signals. For example, a typical second-order Butterworth microfilter may be designed with an insertion loss of about 0.3 dB loss throughout the pass band, which includes the POTS band (about 0-4 kHz) and has a cutoff frequency of about 8 kHz. As those skilled in the art will appreciate, it is highly desirable for this cutoff frequency to be above the POTS signal band and well below the ADSL transmission band (i.e. below about 25 kHz). Given the 8 kHz cutoff frequency, the total attenuation achieved at 25 kHz (the beginning of the DSL band) is, at a maximum, only about 19 dB. This amount of attenuation is generally insufficient in that it allows a significant amount of DSL transmit signal leakage through the filter, and could cause interference with the associated POTS device, particularly if the associated POTS device is a data device, such as a facsimile machine or a data modem.
Further, as with many things, it is desirable to keep the costs of producing the microfilter low. A significant factor in determining the cost of producing a microfilter is the number of components that make up the microfilter. In general, the higher the number of components that make up the microfilter, the higher the cost will be to produce the microfilter. Consequently, it is desirable to keep the component count of a given microfilter design low to keep the production cost low.
Accordingly, a need exists to provide a system and method for preventing energy from DSL and home network signal carriers from reaching voice-band appliances such as telephones, facsimile machines, and 56K modems. Another need exists to provide a system and method for isolating DSL devices and HPNA (Home Phoneline Network Alliance) standard devices from the impedance fluctuations of voice-band appliances. Moreover, an additional need exits to provide a system and method for separating, or isolating, voice-band appliances from DSL and HPNA devices that is robust, inexpensive, and easy to install.